In the construction of large concrete slabs for floors and the like, it is customary to first cast a slab of a size capable of being worked by concrete finishers and then cast further slabs in abutment thereto.
After preparing the ground surface, formwork defining the peripheral edges of the slab is erected and reinforcing mesh is positioned within the formwork surround and supported above the ground surface by spacers known as "bar chairs".
In order to maintain the upper surfaces of adjacent slabs in the same plane, steel reinforcing rods known as "dowel bars" are cast into the initial slab with a portion .projecting from the side walls of that slab. When an adjacent slab is poured, the projecting portion of the dowel bar is encapsulated within the adjacent slab to resist relative movement in an upright plane between the edges of adjacent slabs. As some horizontal movement between adjacent slabs is inevitable due to thermal expansion and contraction, the free end of the dowel bar extending from the edge of a slab is coated with oil or grease to prevent adhesion in the subsequently poured adjacent slab. By allowing the dowel bar to slide within one slab during expansion or contraction, edge fractures are thereby avoided.
Dowel bars for a floor slab construction typically comprise 300 mm to 600 mm lengths of 10 to 30 mm diameter round or square section steel bar arranged at about 600 mm intervals along the edge of a previously cast slab. The dowel bars extend into each adjacent slab from 150 mm to 300 mm.
Although the dowel bars are generally effective for their intended purpose, traditional methods employed for positioning the bars in the initial slab edge are far from satisfactory in that not only are they extremely time consuming and therefore costly. They can lead to considerable frustration on the part of those engaged in slab construction.
Typically, it was customary for specialized formwork erectors to erect a timber formwork surround for a slab to be cast and then to drill apertures for the dowel bars at approximately 300 mm to 600 mm intervals. The apertures provide a neat fit to avoid leakage of concrete through these apertures when the slab is cast Another team specialised in positioning the steel reinforcing mesh then set up the necessary reinforcing structure including the steel dowel bars which protrude outwardly from the apertures in the formwork surrounds.
The main problem associated with this technique is that it is practically impossible to position the dowel bars perfectly parallel in both the upright and horizontal planes. Accordingly. after the slab has cured, extreme difficulty is incurred in removal of the formwork (usually a long timber plank) as a consequence of the non parallel array of protruding ends. To avoid loosening the dowel bars cast into the slab and also to avoid damage to the edge of the slab by attempting to lever the formwork away from the slab, it has become customary to cut the formwork between adjacent dowel bars and slide each segment over the protruding portion of the dowel bar.
Apart from being a costly waste of time and materials. this procedure is frustrating and difficult in view of the fact that the formwork extends to the ground surface thus necessitating a hole to be dug into the ground to enable an electric saw to cut all the way through the formwork.
These prior art problems have been addressed in Australian Patent Application No. 21883/95 and U.S. Pat. Nos. 5005331, 5216862 and 5487249, all of which provide a moulded plastics tubular socket of circular cross section having a closed distal end and an open proximal end with an integral or separate mounting flange to permit attachment to slab casting formwork.
While generally effective for their purpose, the dowel bars socket of the above prior art references do not accommodate slab shrinkage in a transverse direction unless the dowel bar has a lesser diameter than the internal diameter of the dowel bar socket.
In many cases this clearance does not create a problem but in certain high specification applications, the clearance between the dowel bar and Its socket, while allowing transverse shrinkage movement, also allows an unacceptable vertical displacement between a previously cast and subsequently cast slab.
Typically, for injection moulded plastics sockets of say, 300 mm in length, it is necessary to form a divergent taper on both the interior and exterior walls from the closed distal end to the open proximal end to enable removal of the moulded socket from the injection moulding die.
Thus, for a 20 mm dowel bar the internal diameter of the socket bore at the distal end may be 20.0 mm to 20.5 mm whereas the inner bore diameter at the open proximal end will be typically of the order of 21.0 mm to 22.5 mm.
Accordingly unless dowel bars are centred in respective sockets they may be capable of shrinkage movement in a lateral direction of up to 2.5 mm and similarly up to 2.5 mm in a vertical direction.
Generally speaking the loose fit of the dowel bars at the proximal end of the sockets means that they are randomly arranged and substantial variations in movement capacity can exist between adjacent dowel bars. This uneven allowance for lateral shrinkage and unacceptable allowance of relative upright movement between adjacent slabs can give rise to cracking of slabs in the region of the dowel bar as they cure and even after curing if substantial movement is permitted between slabs.